I had the same two-caste system enforced in European office of US company. Legally it was a local company with parent in US. But anyway, "food is only for employees". Funny enough, even student who was there for 20 hours per week and did anything but work was allowed to eat.

This is a cultural thing.

It is a legal and cost thing. Said student is directly employed, the contractors have an employment contract with the agency. And those agencies are used to reduce costs, hence no sponsored, free or discounted food.

No, that's a rationalization. It is because psychopaths are running the place. I have never heard of an employer paying benefit tax on pizzas, and if they did, surely they can bill the consulting firm in some circle if that is the case.